The invention relates to a process for preparing light hydrocarbon fractions and medicinal oil.
Light hydrocarbon fraction, such as naphtha and herosine, may be prepared by two-stage hydrocracking of heavy hydrocarbon oils, such as vacuum distillates of crude mineral oil, as described, e.g., in U.K. Pat. No. 1,400,013. To this end the heavy hydrocarbon oil is hydrocracked in the first stage, the hydrocracked product is separated by distillation into one or more light hydrocarbon fractions and a residue, the residue is hydrocracked in the second stage and the hydrocracked product from the second stage is also separated by distillation into one or more light hydrocarbon fractions and a residue. As a rule, the last-mentioned residue is used as a fuel oil component. The light hydrocarbon fractions separated in the distillation are the desired end product.
In this patent application the term medicinal oils is meant to denote hydrocarbon oils that are odorless and tasteless, which have a color lighter than Saybolt-Color Number +30 and which satisfy the hot acid test according to ASTM-D 565 and the requirement made by FDA No. 121.1146. In the hot acid test according to ASTM-D 565, the oil is treated with concentrated sulphuric acid. The two substances are heated and mixed. Two layers are formed then: an oil and an acid layer. The color of the two layers is tested. To satisfy this test the oil layer must not show any discoloration. The acid layer is allowed to show only little discoloration. This color must not become darker than a reference solution. The requirement made in FDA No. 121.1146 implies that the UV absorption of the oil in the wave-length range of 260-350 nm is at most 0.10. Medicinal oils are used on a large scale in the plastics, cosmetics, food and pharmaceutical industries. They are prepared, for instance, starting fro hydrocarbon fractions with suitable viscosities by treating these fractions successively with large amounts of concentrated sulphuric acid, neutralizing them and treating them with clay. This process has a number of drawbacks. The principal drawback is that of the waste products (acid tar and spent clay) obtained in the process. In connection with the tightening up of legislation concerning environmental hygiene, the disposal of these waste products, which are obtained in this process in considerable amounts, becomes an ever increasing problem. Another drawback of the above-mentioned process is its yield. Depending on the amount of undesired components present in the starting material and removed in the various stages of the refining process, the process may lead to a relatively low yield of medicinal oil. Finally, the process is rather expensive. In order to obviate the above-mentioned drawbacks in the preparation of medicinal oil, it was previously proposed to prepare these oils via a catalytic hydrotreatment, as described e.g. in Netherlands patent application No. 7216510. To this end a hydrocarbon fraction with a suitable viscosity is contacted with a catalyst at elevated temperature and pressure and in the presence of hydrogen. Since in this process the use of large amounts of concentrated sulphuric acid is obviated, and consequently also the formation of considerable amounts of acid tar, the problem of waste products hardly plays a role in this process. Since, furthermore, undesirable constituents present in the starting material are converted in this process into valuable medicinal oil components, instead of being removed from it, this process leads to a considerably higher yield of medicinal oil than the above-mentioned method of preparation using a treatment with a large amount of concentrated sulphuric acid. Finally, the process is considerably less expensive.
Applicants have carried out an investigation to examine to what extend residual fractions obtained in the distillation of the hydrocracked product from the second stage of the above-described two-stage hydrocracking process for the preparation of light hydrocarbon fractions from heavy hydrocarbon oils, can be used for the preparation of medicinal oils by catalytic hydrotreatment. It has been found that when these residual fractions were used as the feed, no medicinal oils could be prepared, not even when use was made of one of the most active catalysts known for the preparation of medicinal oils by hydrotreatment, namely a catalyst containing platinum on a carrier, which carrier consists of 13-15%w alumina, the rest being silica. Although with this catalyst it was possible to prepare, in high yield and under relatively mild conditions, oils that satisfied the requirements for medicinal oils as to taste, odor and color, the oils prepared did not satisfy the hot acid test according to ASTM-D 565 and/or the requirement according to FDA No. 121.1146. Attempts still to obtain a medicinal oil with this catalyst from this feed by increasing the temperature have remained unsuccessful. In addition to a loss in yield by cracking and a higher hydrogen consumption, the use of higher temperatures gave an increasing formation of structures that have an unfavorable effect on the results according to FDA test No. 121.1146 and the hot acid test according to ASTM-D 565.
Continued investigation concerning this subject by Applicants has shown that residual fractions obtained in the distillation of the hydrocracked product from the second stage of a two-stage hydrocracking process for the preparation of light hydrocarbon fractions from heavy hydrocarbon oils, can yet be used in certain cases as the feed for the preparation of medicinal oils by hydrotreatment across a catalyst comprising one or more noble metals of Group VIII on a carrier. For, it has surprisingly been found that in this way medicinal oils can be prepared in high yield from these residues, if the residues originate from the second stage of a hydrocracking process, in which instead of a distillation residue of the reaction product from the first stage, the whole reaction product from the first stage (without ammonia, hydrogen sulphide or light hydrocarbons formed being removed from it is used as the feed for the second stage. In addition to the fact that this embodiment of the two-stage hydrocracking process has the considerable advantage that a valuable product like medicinal oil can be prepared in high yield from a residual by-product having the value of fuel oil, this embodiment of the two-stage hydrocracking process allows the use of simpler equipment than in the procedure with distillation between the stages, because it requires much less fractionation capacity. This means that operation of this embodiment of the two-stage hydrocracking process can be less expensive.